Hardenable Alloy Steels

Extracto:

By far the largest tonnage of alloy steels is of the types containing generally 0.25 to 0.55% C, or less, and usually quenched and tempered for high strength and toughness. Manganese, silicon, nickel, chromium, molybdenum, vanadium, aluminum and boron are commonly present in these steels to enhance the properties obtainable after quenching and tempering.

By far the largest tonnage of alloy steels is of the types
containing generally 0.25 to 0.55% C, or less, and usually
quenched and tempered for high strength and toughness.
Manganese, silicon, nickel, chromium, molybdenum, vanadium,
aluminum and boron are commonly present in these steels to
enhance the properties obtainable after quenching and
tempering.

These alloy steels are ordinarily quench-hardened and
tempered to the level of strength desired for the
application. Even though the strength level at which the
steel is used may be as low as, or lower than, that which
could be achieved by the microstructure (fine pearlite or
upper bainite) developed by a simple cooling from forging or
normalizing temperature, the steels are quench-hardened and
tempered, indirectly reflecting the engineering and economic
basis of the demand for this type of steel.

The microstructure (tempered martensite or bainite) produced
by quenching and tempering these alloy steels is characterized
by a greater toughness or capacity to deform without rupture
at any strength level. Similarly, under the adverse state of
stress below a notch in bending, the tempered martensite may
flow considerably at a testing temperature far below that at
which a pearlitic steel of equal strength would break in a
brittle manner; the Charpy or Izod values are thus improved.
The basic phenomenon of developing this favorable
microstructure by heat treatment is manifested in plain
carbon steels, but only in small sections; thus the most
important effect of the alloying elements in these steels is
to permit the attainment of this microstructure, and the
accompanying superior toughness in larger sections.

Alloying Elements Dissolved in Austenite. The general
effect of elements dissolved in austenite is to decrease the
transformation rates of the austenite at subcritical
temperatures. The only one among the common alloying elements
to behave exceptionally in this regard is cobalt. Since the
desirable products of transformation in these steels are
martensite and lower bainite, formed at low temperatures,
this decreased transformation rate is essential; it means
that pieces can be cooled more slowly, or larger pieces can
be quenched in a given medium, without transformation of
austenite to the undesirable high-temperature products,
pearlite or upper bainite.

This function of decreasing the rates of transformation,
and thereby facilitating hardening to martensite or lower
bainite, is known as hardenability and is the most important
effect of the alloying elements in these steels. Thus, by
increasing hardenability the alloying elements greatly extend
the scope of enhanced properties in hardened and tempered
steel to the larger sections involved in many applications.

The several elements commonly dissolved in austenite prior to
quenching, increase hardenability in approximately the
following ascending order: nickel, silicon, manganese,
chromium, molybdenum, vanadium and boron. The effect of
aluminum on hardenability has not been accurately evaluated.
But at 1% Al, as used in "nitralloy" steels, the
effect on hardenability seems to be relatively small. Further,
it has been found that the addition of several alloying
elements in small amounts is more effective in increasing
hardenability than the addition of much larger amounts of one
or two.

In order to increase hardenability effectively, it is
essential that the alloying elements are dissolved in
austenite. The steels containing the carbide-forming elements
- chromium, molybdenum and vanadium - require special
consideration in this respect. These elements are present
predominantly in the carbide phase of annealed steels, and
such carbides dissolve only at higher temperatures and more
slowly than iron carbide. Since the basic function of the
alloying elements in these steels is to increase
hardenability, the selection of steel and the choice of
suitable austenitizing conditions should be based primarily
on the assurance of adequate hardenability. More than
adequate hardenability is rarely a disadvantage, except in
cost.

Alloying Elements in Quenching. Since the sections
treated are often relatively large, and since the alloying
elements have the general effect of lowering the temperature
range at which martensite is formed, the thermal and
transformational stresses set up during quenching tend to be
greater in these alloy steel parts than those involved in
quenching the necessarily smaller sections of plain carbon
steels. In general, this means greater distortion and risk of
cracking.

The alloying elements, however, have two functions that tend
to offset these disadvantages. The first and probably the
most important of these functions is that of permitting the
use of lower carbon content for a given application. The
decrease in hardenability accompanying the decrease in carbon
content may be offset very readily by the hardenability
effect of the added alloying elements, and the lower-carbon
steel will exhibit a much lower susceptibility to quench
cracking. This lower susceptibility results from the greater
plasticity of the low-carbon martensite and from the
generally higher temperature range at which martensite is
formed in the lower-carbon materials. Quench cracking is
seldom encountered in steels containing 0.25% C or less, and
the susceptibility to cracking increases progressively with
increasing carbon content.

The second function of the alloying elements in quenching is
to permit slower rates of cooling for a given section,
because of increased hardenability, and thereby generally to
decrease the thermal gradient and, in turn, the cooling
stress. It should be noted, however, that this is not
altogether advantageous, since the direction, as well as the
magnitude, of the stress existing after the quench, is
important in relation to cracking.

In order to prevent cracking, the surface stresses after
quenching should be either compressive or at a relatively low
tensile level and, under certain circumstances, lowering the
cooling rate may lead to increased tensile stresses at the
surface, thus increasing the tendency to crack. In general,
though, unless a study of the particular piece being quenched
indicates that it falls within this category of increased
susceptibility with decreased quenching rates, the use of a
less drastic quench suited to the hardenability of the steel
will result in lower distortion and greater freedom from
cracking.

Furthermore, the increased hardenability of these alloy
steels may permit heat treatment by "austempering" or
"martempering", and thereby the level of adverse residual
stress before tempering may be held to a minimum. In
"austempering", the work piece is cooled rapidly to a
temperature in the lower bainite region and is allowed
thereafter to transform, completely at some chosen
temperature. Since this transformation occurs at a relatively
high temperature and proceeds rather slowly, the stress level
after transformation is quite low and distortion is held to a
minimum.

In "martempering", the piece:

is cooled rapidly at the surface to a temperature that
permits very little martensite to form, if any;

is equalized at this temperature; and

is then cooled slowly so that transformation throughout
the whole section occurs more or less simultaneously, thereby
holding transformational stresses at a very low level and
minimizing distortion and danger of cracking.

Alloying Elements in Tempering. Hardened steels are
softened by reheating, but this effect is not the one
actually sought in tempering. The real need for increasing
the capacity of the piece is to flow moderately without
fracture, and this is inevitably accompanied by a loss of
strength. Since the tensile strength is very closely related
to hardness in this class of steels, as heat-treated, it is
satisfactory to follow the effects of tempering by measuring
the Brinell or Rockwell hardness. The statistical mean of the
relationships among Brinell hardness, tensile strength and
yield strength, is shown in Fig. 1, drawn from many data.

Figure 1.

Figure 2-4 shows the softening pattern of nine 0.45% C
steels with increase in the tempering temperature, for one
hour. Somewhat shorter or longer intervals at temperature
would show little difference in hardness values.

Figure 2.

Figure 3.

Figure 4.

As a first approximation, the softening pattern of steels
similar but differing in carbon content through the range
from 0.25 to 0. 55% C may be estimated from Fig. 3.

Figure 5.

This illustrates the general effect of carbon content on this
softening pattern in terms of Rockwell C hardness units to be
added to or subtracted from the 0.45% C value at
different levels of tempering temperature, but accurate data
that would permit a strictly quantitative relationship of
this type are not available.

The effect of carbon content on the hardness of the tempered
steels is much greater for the lower tempering temperatures
than for 1200oF (649oC) and higher, and
that the effect likewise decreases when there is more than
0.50% C. Figures 2,3,4 and 5, used together, make it
possible to estimate the strength from a given tempering
treatment applied to a given type of steel.

The general effect of the alloying elements is to retard the
softening rate, so these alloy steels will require a higher
tempering temperature to obtain a given hardness than carbon
steel of the same carbon content. However, the individual
elements show significant differences in the magnitude of
their retarding effect. Nickel, silicon, aluminum and, to a
large extent, manganese, which have little or no tendency to
occur in the carbide phase, and merely remain dissolved in
ferrite, have only a minor effect on the hardness of the
tempered steel - an effect that would be expected from the
general pattern of solid-solution hardening.

Chromium, molybdenum and vanadium, on the other hand, which
migrate to the carbide phase when diffusion is possible,
bring about a retardation of softening, particularly at the
higher tempering temperatures. These elements do not merely
raise the tempering temperature; when they are present in
higher percentages, the rate of softening is no longer a
continuous function of the tempering temperature. That is,
the softening curves for these steels will show a range of
tempering temperature in which the softening is retarded or,
with relatively high alloy content, in which the hardness may
actually increase with increasing tempering temperature. This
characteristic behavior of the alloy steels containing the
carbide-forming elements is known as "secondary hardening"
and results presumably from a delayed precipitation of fine
alloy carbides.

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